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Ever since 3-D movies burst onto the scene, sometimes seeing things in 1-D just won’t cut it. And the same goes for ab training—work your abs in only one dimension, and you cheat yourself out of your full, six-pack potential.

"Most exercises only work in a single plane," says Irv Rubenstein, PhD, exercise physiologist and founder of S.T.E.P.S., a fitness facility in Nashville, TN. "Incorporating 3-D moves includes muscles such as the obliques, which provide a 'girdling' effect, pulling the waist inward and enhancing the definition of the rectus abdominis to give you that six-pack appearance."

To understand how to incorporate this principle, you'll need to visualize the planes of the body. Backward and forward motions, such as crunches and reverse crunches, work in the sagittal plane, which divides the body into right and left halves. Lateral movements work within the frontal plane, which separates the body into front and back. And twisting movements occur in the body's transverse plane, which divides the body into upper and lower halves.